Creation in the blink of an eye

The Crab Nebula in an image recorded by the Hubble Space Telescope. Photograph: Nasa

Cosmologists are agreed that the universe began with a big bang. Direct evidence comes from the fact that the universe is still expanding today, with clusters of galaxies flying apart from each other at immense speeds. In the 1960s, it was discovered that the entire universe is bathed in heat radiation, which is readily explained as the fading afterglow of the great initial explosion. The radiation has travelled through space almost undisturbed for billions of years, so it provides a snapshot of what the universe was like in its infancy. By scrutinising the cosmic heat radiation in detail using satellites, scientists have been able to pin down many of the universe's vital statistics to remarkable accuracy, for example its age, now put at 13.72bn years.

Amazingly, it is possible to fleetingly re-create in the laboratory the physical conditions that prevailed in the early universe. This is done using giant accelerator machines, sometimes called atom smashers, to direct subatomic particles into head-on collisions at enormous energies. By studying the aftermath, physicists have built up an accurate picture of how matter would have behaved as early as a billionth of a second after the big bang.

The epoch before about a billionth of a second, however, remains murky territory, with plenty of scope for disagreement. A key cosmological observation that needs explaining is the extraordinary large-scale uniformity of the universe, as manifested in the even distribution of heat radiation across the sky. This suggests that the big bang erupted with the same degree of vigour in all places and in all directions, quite unlike a conventional explosion. The favoured explanation is that the universe abruptly leapt in size by a huge factor during the first split second, thus smoothing everything out - a process dubbed "inflation". Following inflation, the expansion persisted, but at a diminishing rate, as the gravitational pull of all the cosmic material served as a brake. The inflationary theory makes definite predictions about the geometry and rate of expansion of the universe, predictions that seem to accord well with observations.

The universe is not, however, completely smooth, or we would not be here to observe it. The cosmic heat radiation betrays tiny variations in temperature, reflecting slight irregularities in the density of the primordial gases. The over-dense regions eventually developed into the clusters of galaxies we see today. Nobody knows what caused these irregularities, but they could be due to the effects of quantum physics operating during the period of inflation. It is a remarkable thought that the structure of the universe on a scale of hundreds of millions of light years could well be a manifestation of minute quantum fluctuations, enormously distended and writ large in the sky.

Whatever the success of the big bang theory in explaining the key observed features of the universe, it is clearly incomplete. People always want to know what came before the big bang. Why did it happen at all? Here physical theory merges with philosophy and even theology. In the simplest model of the universe, based on Einstein's general theory of relativity, the big bang is the origin of time and space, as well as matter. If time itself began with the big bang, then questions about what happened before it, or what caused it, cease to have meaning. But to simply accept the origin of the universe as an unexplained brute fact is deeply unsatisfying.

In the 1960s, the ultimate origin of the universe was regarded as lying beyond the scope of science altogether, but today there are many attempts to explain it using physical theory, most often by appealing to quantum processes. If the big bang was indeed a natural event, then presumably nothing could prevent it from happening more than once. This suggests there may be many big bangs scattered throughout space and time, each producing an expanding universe of some sort. Possibly the entire assemblage of universes - often dubbed "the multiverse" - is eternal, even though each individual universe undergoes a life cycle of birth, evolution and perhaps death.

In a popular version of the multiverse theory, called eternal inflation, universes "nucleate" like bubbles in a liquid, and although each bubble universe may expand explosively fast, different bubbles are conveyed apart by unending inflation in the overall matrix of space faster than the bubbles themselves expand. As a result, the different universes rarely collide.

Underlying all cosmological theory is the tacit assumption that the universe as a whole obeys physical laws that themselves enjoy independent existence. For example, the laws of quantum physics that may permit a big bang to occur spontaneously must in some sense "already exist" if they are to account for the universe's origin. This prompts the question of where those laws come from, and why they have the form that they do. In particular, scientists would like to know why the laws provide a universe-generating mechanism, and why, in our own universe at least, they permit life.

Here scientists are sharply split. Some think the laws of physics can vary from one universe to another, allowing life-bearing universes to occur only as a rare fluke. Others think there is a single universe upon which a unique set of laws was imprinted at its birth; the fact that these laws are bio-friendly is dismissed as an incidental bonus. Yet others think the laws and the universe emerged together. Even multiverse proponents divide into those who think the assemblage of universes is eternal, and those who believe there was an ultimate origin of all things.

Everyone agrees, however, that many of the deepest questions about our cosmic origins cannot be answered within the framework of existing physical theory. Hopes are pinned on a final unified theory that will merge all of physics into a single superlaw. Only then might we be able to answer the most fundamental question of all: why there is something rather than nothing.

·Paul Davies is a theoretical physicist, cosmologist, astrobiologist, author and broadcaster based at Arizona State University